This invention relates generally to condition-responsive controls and more particularly to a condition-responsive switch mechanism and a method for assembly of such a mechanism.
Switches that are responsive to temperature changes, commonly known as thermostats or cold controls, are used in refrigeration appliances, such a refrigerators and freezers, to control the temperatures therein. These thermostats regulate the switching cycle of the refrigeration compressor in response to the temperature of the air contained at some location within the appliance. When the temperature exceeds a certain "turn-on" point, the switch contacts are closed and the compressor is switched on to cool the appliance. When the temperature drops below a certain "turn-off" point, the switch contacts are opened and the compressor is switched off. Examples of thermostats for refrigeration appliances are set forth in U.S. Pat. No. 3,065,320 (Cobean), U.S. Pat. No. 3,065,323 (Grimshaw), U.S. Pat. No. 3,648,214 (Slonneger), U.S. Pat. No. 4,490,708 (Thompson et al.), and U.S. Pat. No. 5,142,261 (Fuller et al.). All of these patents are assigned to General Electric Company, the assignee of the present application, and their disclosures are expressly incorporated herein by reference.
Thermostats of the type to which this invention relates typically employ a bellows communicating with a capillary tube in thermal contact with the location to be cooled. Expansion and contraction of a gas within the capillary tube and bellows causes corresponding expansion and contraction of the length of the bellows. The motion of the bellows is transmitted via an actuator to a switch element such as a bistable spring switch element which is capable of snapping between two stable positions, one of which closes a circuit and activates the compressor to cool the appliance and the other of which opens the circuit to deactivate the compressor. The spring switch element is fixed to one circuit element and extends outwardly toward another circuit element and carries a electrical contact on its free end. In the circuit open position of the spring switch element, the spring switch element is spaced away from the other circuit element. In the circuit closed position, the contact on the spring switch element engages a contact fixed to the other circuit element and the circuit is completed. Snapping of the spring switch element is controlled by the actuator in the thermostat which presses against the spring switch element with a force increasing with the increase in temperature above the set point detected within the appliance. Eventually, the force reaches a switch point at which the spring switch element snaps from one position to another to open or close the circuit.
It has been found that as the actuator gradually approaches the switch point, the force with which the spring switch element urges its electrical contact against the fixed contact on the other circuit element is substantially reduced. In fact, the contact urging force goes substantially to zero as the actuator approaches the switch point. At low contact forces, the presence of particulate matter on either contact can cause a loss of electric connection. Vibrations in the appliance can also cause the connection to rapidly break and reconnect near the switch point, resulting in undesirable "chattering" of the compressor. These conditions may also lead to arcing between the contacts which can damage the contacts and change the operating characteristics of the thermostat.
It is desirable to have a single thermostat which is capable of operating different kinds of appliances without substantial modification. One important difference between different types of appliances is the need for the thermostat to have different sensitivities. For instance, one manufacturer may have a product for which a 10.degree. F. variation in temperature from set point is desired, another may permit only a 5.degree. F. variation and so on. One convenient way of achieving different sensitivities in the same thermostat is to vary the spacing between the fixed contact and the movable contact in the circuit open position. However, it has been found that the operability of each bistable spring switch element, having its own particular size and geometry, is very dependent upon contact spacing. Some spring switch elements will operate properly only when the contact gap is relatively wide (causing the thermostat to permit a relatively wide variation in temperature), others only when the gap is relatively narrow, and others only when the gap is somewhere in between. Thus, the applicability of any given thermostat to different appliances requiring different control sensitivities has heretofore been limited. Moreover in some applications, there is a demand for very silent operation of the appliance. Thus, any snapping or clicking noise which occurs as a switch element opens or closes is undesirable.
Mass production of thermostats is greatly facilitated by automation of assembly where possible. Presently, the capillary tubes are assembled with the bellows early in the process. The capillary tubes hang away from the thermostat, tend to become entangled in the machinery, and generally make automated handling difficult. In addition, the length of the capillary tube in the finished thermostat will be different depending upon the particular application and manufacturer who will use the switch. Typically, final sizing of the capillary tube occurs near the end of the assembly process by cutting the tube down to size, thereby wasting material. Calibration of the switch mechanism with the capillary tube attached is somewhat time consuming because the mass of refrigerant or air in the capillary tube slows down the reaction of the bellows to the calibrating stimulus.
The accuracy of the thermostat to turn on and off the compressor at the desired temperature settings for the appliance is dependent in part upon the purity of the refrigerant in the bellows and capillary tube. The more pure the refrigerant, the more closely its expansion and contraction in response to temperature behaves in an ideal, predictable fashion. Presently, thermostats are charged with vaporous refrigerant from the container in which the refrigerant is supplied to the factory. Air and other contaminants present in the vapor in various amounts can cause the thermostats to operate outside of specification for the refrigerant. Thus, the operating characteristics of one thermostat may be somewhat different than the next although both are manufactured identically.
Although the spring switch element of the present invention described hereinafter, is particularly adapted for use in a thermostat, it is also believed to be useful in other condition responsive switching devices such as one directly responsive to detected position or mechanical pressure.